Homogeneous chemical reactions, processes involving

As mentioned in Chapter 1, Section 2.2, it is quite common that a heterogeneous electron transfer process is complicated by homogeneous chemical reactions that involve the species Ox and/or Red. In this light, the chemical complications are classified as ... 

The nature of electrode processes can, of course, be more complex and also involve phase fonnation, homogeneous chemical reactions, adsorption or multiple electron transfer [1, 2, 3 and 4],... 

One of the most intriguing aspects of electrochemistry involves the homogeneous chemical reactions that often accompany heterogeneous electron-transfer processes occurring at the electrode-solution interface. The addition or removal of an electron from a molecule generates a new redox state, which can be chemically reactive. A variety of mechanisms, some of which involve complicated sequences of electrode and chemical reactions, have been characterized. Several of the more common mechanisms with examples of applicable chemical systems are described next. More examples are given in Chaps. 21 and 23. 

Many interesting processes occurring at the liquid/liquid interface involve coupled homogeneous chemical reactions. In principle, electrochemical methods used for probing complicated mechanisms at metal electrodes (61) can be employed at the ITIES. However, many of these techniques (e.g., rotating ring-disk electrode or fast-scan cyclic voltammetry) are hard to adapt to liquid/liquid measurements. Because of technical problems, few studies of multistep processes at the ITIES have been reported to date (1,62). 

By process, we mean what occurs inside the reactor. If the material in the reactor is single phase and homogeneous, then the process is a reaction. Such a reaction can occur in a batch, a semi-batch, or a continuous reactor, depending upon our design. However, if the material in the reactor is multiphase, e.g., gas—liquid or two immiscible liquids, then it is a process. In other words, conversion of reactant to product involves more than chemical reaction it involves multiple steps, some of which are physical, such as diffusion across a phase boundary. If diffusion across a phase boundary or diffusion through one of the phases in the reactor is slower than the chemical reaction, then we define the process as diffusion rate limited. If physical diffusion occurs at a much higher rate than chemical reaction, then we define the process as reaction rate limited. ... 

Four processes are usually involved in co-reactant ECL systems (as presented in Table 2.1) such as (a) redox reactions at electrode, (b) homogeneous chemical reactions, (c) excited-state species formation, and (d) light emission. Two types of redox reactions, namely heterogeneous and homogeneous redox reactions of co-reactants, are possible, which depend on the redox potential of the co-reactant and nature of the working electrode [2]. 

As will now be clear from the first Chapter, electrochemical processes can be rather complex. In addition to the electron transfer step, coupled homogeneous chemical reactions are frequently involved and surface processes such as adsorption must often be considered. Also, since electrode reactions are heterogeneous by nature, mass transport always plays an important and frequently dominant role. A complete analysis of any electrochemical process therefore requires the identification of all the individual steps and, where possible, their quantification. Such a description requires at least the determination of the standard rate constant, k, and the transfer coefficients, and ac, for the electron transfer step, or steps, the determination of the number of electrons involved and of the diffusion coefficients of the oxidised and reduced species (if they are soluble in either the solution or the electrode). It may also require the determination of the rate constants of coupled chemical reactions and of nucleation and growth processes, as well as the elucidation of adsorption isotherms. A complete description of this type is, however, only ever achieved for very simple systems, as it is generally only possible to obtain reliable quantitative data about the slowest step in the overall reaction scheme (or of two such steps if their rates are comparable). 

The electrochemical oxidation and reduction reactions of organic molecules often comprise complex sequences of electrochemical and chemical steps. In addition to the electron transfer step, coupled homogeneous chemical reactions are frequently involved. The study of chemical reactions or intermediates which are not a major species (in bulk solution ) but are only formed by a chemical process, after an electrochemical reaction, has become a major application of several techniques e.g. cyclic... 

Generally, the electrochemical reaction is a heterogeneous, multi-step process. These steps can be consecutive or parallel they can include homogeneous chemical reactions, transport processes, adsorption phenomena, crystals nucleation and growing, as well as formation of new phases, etc. However, one essential step, always required to occur in the electrochemical reaction is the electron transfer through the electrolyte solution-electrode phase boundary. Thus, the electronic conductivity of at least one phase is crucial for the reaction to proceed. The overall reaction can involve several electrons the electrons being transferred simultaneously or stepwise. In the latter case, other steps sometimes take place between the electron transfer steps. 

Most of the chemical reactions presented in this book have been studied in homogeneous solutions. This chapter presents a conceptual and theoretical framework for these processes. Some of the matters involve principles, such as diffusion-controlled rates and applications of TST to questions of solvent effects on reactivity. Others have practical components as well, especially those dealing with salt effects and kinetic isotope effects. 

A more interesting situation is found when the homogeneous redox reaction is combined with a chemical reaction between the electrocatalyst and the substrate. In this case, the catalytic process is called chemical catalysis. 3 This mechanism is depicted in Scheme 2 for reduction. The coupling of the electron transfer and the chemical reaction takes place via an inner-sphere mechanism and involves the formation of a catalyst-substrate [MC-S] complex. Here the selectivity of the mechanism is determined by the chemical step. Metal complexes are ideal candidates... 

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